Systems employing wireless to the network (WTTN) can now deliver the same data rate as fiber to the network (FTTN) solutions. These systems provide point to multi-point connections at giga-bit rate levels. However, an advantage of the wireless approach to the fiber approach is that WTTN can achieve a cost of approximately one quarter that of FTTN.
In one example system implementation, a network access point (NAP) is provided which serves a set of local access points (LAP) via wireless point-to-point connection. Each local access point in turn provides service to a localized set of customers. A challenge for WTTN is to provide constant bit rate transmission to each LAP. One reason for this is that wireless links are always environment dependent.
Various multi-beam transmission schemes exist. These include fixed beam methods, and adaptive beam forming methods.
With the fixed beam method, a set of fixed beams is used to increase simultaneous transmission receivers, and to achieve high data rate transmission with a certain number of beams, M. An example with M=18 is illustrated in FIG. 1. The fixed beam method can be employed for either transmission or reception. To achieve a good performance with a fixed beam system, the best beam is selected for each receiver in both the transmit and receive directions, resulting in the highest signal-to-noise ratio (SNR) for each receiver. The fixed beams can be generated, for example, by a Rotman lens. The beam pattern of FIG. 1 is generated by a sectorized transmitter having three sectors. Typically, the beams generated at the center of the sector will be the strongest while the beams closer to the edge of the sector will have less strength.
In the fixed beam method described with reference to FIG. 1, there are two problems. The first problem is that the number of receivers assigned into each beam is not balanced resulting in unequal throughput for each receiver. The second problem occurs when a receiver is located near the boundary of two beams. This results in a dead spot where the receiver cannot be served with a required minimum data rate.
The adaptive beamforming technique uses a spatial processing with an antenna array, requiring an optimum combining so as to improve system performance. The optimum beamforming combining performed by an adaptive array antenna optimizes the beamformer response so that the output contains minimum contributions due to noise and signal arriving from directions other than the desired signal direction. An example beamforming array antenna with 3-sectorization each employing six antenna elements with half wavelength spacing is illustrated in FIG. 3. The problem for adaptive beamforming is that the beams formed by the antenna array could collide with each other when two receivers are closely located, resulting an extremely low SNR in receiver.
Typically for either of the above methods, a random or round robin scheduling method is employed to determine which receivers to schedule in a given transmission interval. The conventional round robin or random scheduling solutions fail to meet guaranteed CBR (constant bit rate) requirements. It would be desirable to have scheduling methods which improve upon the performance of simple round robin or random scheduling.